Trees have internal thermostat

Trees from the linkurl:Caribbean;http://www.the-scientist.com/article/display/53628/ to linkurl:Canada;http://www.the-scientist.com/article/display/15054/ maintain a constant leaf temperature regardless of the ambient air temperature, according to a new study by researchers at the University of Pennsylvania. The findings could dramatically alter interpretations of data used to approximate past climate from the composition of tree rings, the researchers say.

By Bob Grant | June 11, 2008

Trees from the linkurl:Caribbean;http://www.the-scientist.com/article/display/53628/ to linkurl:Canada;http://www.the-scientist.com/article/display/15054/ maintain a constant leaf temperature regardless of the ambient air temperature, according to a new study by researchers at the University of Pennsylvania. The findings could dramatically alter interpretations of data used to approximate past climate from the composition of tree rings, the researchers say.
Scientists who measure cellulose composition in tree rings to reconstruct past climate conditions have long assumed that the temperature of linkurl:photosynthesizing;http://www.the-scientist.com/news/20021122/02/ leaves was essentially the same temperature as the surrounding air, said linkurl:Brent Helliker,;http://www.bio.upenn.edu/faculty/helliker/ a U. Penn ecologist and coauthor of the study. "Trees aren't good thermometers," said linkurl:Christopher Field,;http://fsi.stanford.edu/people/christopherfield/ a global ecologist at the Carnegie Institution for Science who wasn't involved with the study. Field said that instead of looking at tree rings as faithful records of ambient temperature, modelers now had to introduce a correction factor into the equation going from growth and photosynthesis to climate.
"When you look at tree rings," Helliker said, "you need to take the actual physiology and ecology of the tree into account."
Helliker and his coauthor linkurl:Suzanna Richter,;http://www.sas.upenn.edu/earth/suzier.html a postdoc at U. Penn, sampled cellulose from 39 tree species across 50 degrees of latitude, from broadleaf trees in the Caribbean to pines and firs in the boreal forests of Northern Canada. The two then measured the ratio of linkurl:isotopic oxygen,;http://www.the-scientist.com/news/display/22944/ a proxy for growth, in the trees' cellulose and found that photosynthesis was occurring when the leaves were about 21˚ C, irrespective of latitude or ambient temperature.
"When I did [the calculations] with the full dataset and actually solved for what the temperature was, my jaw dropped," Helliker said. The result suggested that the leaf were actively maintaining leaf temperature homeostasis.
Rather than regulating their temperature through physiological mechanisms, as mammals do, trees seem to utilize structural and morphological attributes, Helliker explained. For example, linkurl:conifer trees,;http://www.the-scientist.com/news/display/23016/ which have needle-like leaves that lose heat readily, pack branches and needles together in the canopies of linkurl:boreal forests;http://www.the-scientist.com/article/display/54053/ to retain heat through cold winters.
"It's really important for the climate community to realize that plants are strategists," said Field. "The things we use as proxies for past climate conditions are complicated systems. They're not just passively soaking up what is dished out."
linkurl:William Smith,;http://www.wfu.edu/biology/faculty/smithwk.htm a Wake Forest University biophysical ecologist, said that the finding bridges the worlds of climate modelers and plant physiologists by injecting thermal biophysics at the leaf level into broader climate modeling and prediction. "This contribution is important because a lot of ecosystem modeling types don't really have a grasp of this because they're working on a higher spatial scale," Smith said. "This will bring more ecosystem people down to the leaf level."
Helliker said that his correction to past models that assumed a tight coupling of air temperature to leaf temperature can not only improve reconstructions of past climate, but may be able to help scientists better predict linkurl:future climate change;http://www.the-scientist.com/article/display/54049/ and how Earth's vegetation may respond. "Now we have the potential opportunity to pull out how the trees themselves respond to both year to year weather variation but also the warming that we've seen over the last hundred years," he said.

Comments

If true, I have to say, "Wow." This is huge. But since they didn't actually take the temperature of the leaves, are they really sure this is temperature regulation, and not something else that works out in their equations like that? \n\nI have walked in pine forests when it's cold and touched the needles and they were cold - not thinking about photsynthesis at the time. I have touched leaves in deciduous trees at various times of year and while they seemed cooler than the air in summer (which makes sense) they definitely seemed cold in colder weather. \n\nI think they need to do a study of actual temperature, because I think there is something balancing out in their equations.

The authors never claimed that the leaves of live trees cannot get preyyy cold, or pretty hot.\n\nAll they claimed is that the leaves photosynthesize only at about 21 degees, and that trees have various "strategies" that give many of their green leaves a good chance of often achieving such a temperature.

Measuring the temperature of leaves with your fingers probably isn't going to get you too far. Fingers are probably better at measuring the heat capacity of a cold object rather than its absolute temperature. Try touching a car left out in very cold weather; the metal will feel really cold but the tyres not so, yet they are both at the same temperature.

There is often confusion between constancy(relative) and regulation. I am not sure that this topic is devoid of this confusion.If my blood pressure drops and I faint and fall down, return of the blood supply to the brain by fainting cannot be considered to be a regultion but merely incidental to a parallel process that relates to posture. Similarly, if the pine leaves are bunched up and probably diminish air circulation, it can hardly be considered to a regulation since it is derived from its structure. A thermostat is far fetched because it invokes an active process-dependent loop that is normative. None is clear here.

Interesting. The article could have picked better metaphores, but the point got across: something else is happening that we are not including in our equations. Until we find out what that is, we cannot consider it very safe to act as though we understand tree rings or tree behavior in response to temperature.